The present invention relates to a voltage controlled oscillator (VCO) circuit suitable for use in a phase locked loop to create an analog phase locked loop with high noise rejection.
On chip VCO's are usually implemented using a multivibrator circuit where the oscillation is controlled by charging and discharging an on-chip capacitor. A common application for such a circuit is in an optical high speed data link, which usually house a serializer VCO and a deserializer VCO on the same module. These VCO's are especially sensitive to power supply noise, other VCO's and data patterns which have the same frequency component as the VCO operating frequency but are asynchronous with each other. This type of noise is referred to as near frequency noise. Enough noise of this kind can override the control voltage input to the VCO and cause the phase locked loop, of which the VCO is a part, to lose phase lock. Noise which contains odd harmonic frequencies of the fundamental will also affect the VCO although the sensitivity is not as dominant as the fundamental frequency.
A previous approach to reduce noise sensitivity was to use passive filtering techniques to filter the high frequency noise using R,L,C networks on the module or external to the module. This approach does not have full frequency band rejection and uses a large number of components.
Another approach is to use a digital phase locked loop, which inherently has better noise immunity. For applications such as high speed (gigahertz) clock recovery the digital phase locked loop cannot be implemented at the speeds desired with present CMOS technology.
It is an object of the present invention to provide a noise immune VCO that can be implemented on-chip and which can supply sufficient transient current and unimpaired AC performance.
It is a further object of the present invention to provide a VCO for use in an analog phase locked loop which is tolerant of near frequency noise.
It is another object of the present invention to provide a VCO for use in an analog phase locked loop which is more tolerant of harmonic noise injected into the power supply of the VCO from other external circuits, acting as noise sources.